MP4561 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER

MP4561 Rev. 1.0

www.MonolithicPower.com

8

11/5/2012

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© 2012 MPS. All Rights Reserved.

During operation, the minimum COMP voltage is

clamped to 0.9V and its maximum is clamped to

2.0V. COMP is internally pulled down to GND in

shutdown mode. COMP should not be pulled up

beyond 2.6V.

Internal Regulator

Most of the internal circuitries are powered from

the 2.6V internal regulator. This regulator takes

the VIN input and operates in the full VIN range.

When VIN is greater than 3.0V, the output of the

regulator is in full regulation. When VIN is lower

than 3.0V, the output decreases.

Enable Control

The MP4561 has a dedicated enable control pin

(EN). With high enough input voltage, the chip

can be enabled and disabled by EN which has

positive logic. Its falling threshold is a precision

1.2V, and its rising threshold is 1.5V (300mV

higher).

When floating, EN is pulled up to about 3.0V by

an internal 1µA current source so it is enabled.

To pull it down, 1µA current capability is needed.

When EN is pulled down below 1.2V, the chip is

put into the lowest shutdown current mode.

When EN is higher than zero but lower than its

rising threshold, the chip is still in shutdown

mode but the shutdown current increases slightly.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage. The UVLO rising threshold is

about 3.0V while its falling threshold is a

consistent 2.6V.

Thermal Shutdown

Thermal shutdown is implemented to prevent the

chip

from

operating

at

exceedingly

high

temperatures. When the silicon die temperature

is higher than its upper threshold, it shuts down

the whole chip. When the temperature is lower

than its lower threshold, the chip is enabled again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor. This floating

driver has its own UVLO protection. This UVLO’s

rising threshold is 2.5V with a hysteresis of

300mV. The driver’s UVLO is soft-start related. In

case the bootstrap voltage hits its UVLO, the

soft-start circuit is reset. To prevent noise, there

is 20µs delay before the reset action. When

bootstrap UVLO is gone, the reset is off and then

soft-start process resumes.

The bootstrap capacitor is charged and regulated

to about 4V by the dedicated internal bootstrap

regulator. When the voltage between the BST

and SW nodes is lower than its regulation, a

PMOS pass transistor connected from VIN to

BST is turned on. The charging current path is

from VIN, BST and then to SW. External circuit

should provide enough voltage headroom to

facilitate the charging.

As long as VIN is sufficiently higher than SW, the

bootstrap capacitor can be charged. When the

power MOSFET is ON, VIN is about equal to SW

so the bootstrap capacitor cannot be charged.

When the external diode is on, the difference

between VIN and SW is largest, thus making it

the best period to charge. When there is no

current in the inductor, SW equals the output

capacitor.

At higher duty cycle operation condition, the time

period available to the bootstrap charging is less

so the bootstrap capacitor may not be sufficiently

charged.

In case the internal circuit does not have

sufficient voltage and the bootstrap capacitor is

not charged, extra external circuitry can be used

to ensure the bootstrap voltage is in the normal

operational region. Refer to External Bootstrap

Diode

in Application section.

The DC quiescent current of the floating driver is

about 20µA. Make sure the bleeding current at

the SW node is higher than this value, such that:

O

O

V

I20μA

(R1 R 2)

+>

+

Current Comparator and Current Limit

The power MOSFET current is accurately sensed

via a current sense MOSFET. It is then fed to the

high speed current comparator for the current

mode control purpose. The current comparator

takes this sensed current as one of its inputs.

When the power MOSFET is turned on, the